The present invention relates to a system for detecting a combustion condition which is available for controlling the ignition timing of an engine.
It is desirable to control the idling speed of the engine to a low speed in accordance with requirements for fuel economy and reduction of engine noise. However, a low idling speed causes fluctuation of the engine speed and hence reduction of starting characteristics of a motor vehicle driven by the engine. It is generally deemed that the differences between the combustion conditions of respective cylinders of the engine cause the fluctuation of the idling speed. The fluctuation of the combustion conditions are caused by the following differences.
1) The difference in distribution rate of intake air caused by a complicated configuration of an intake manifold and by interference between the intake-air distributed to respective cylinders;
2) The difference between combustion temperatures in respective cylinders, which is caused by a disposition of a coolant passage;
3) The differences in capacity of a combustion chamber, shape of a piston, etc., which are caused by the manufacturing tolerance;
4) The difference in air-fuel ratio in respective cylinders which is caused by the difference of the injected fuel amount.
It will be understood that the idling speed becomes constant by rendering the above described combustion conditions uniform.
Japanese Patent Application Laid-Open 62-55461 discloses a system for detecting combustion conditions in respective cylinders. The system has pressure sensors provided at every cylinder for detecting pressure in the cylinder. The combustion condition of each cylinder is estimated from the peak pressure of the detected pressures by each pressure sensor.
However, the system is costly because the pressure sensor is comparatively expensive. In addition, a cylinder head of the engine must be drilled for attaching the pressure sensor. Consequently, it is difficult to apply the system to conventional engines.
On the other hand, Japanese Patent Application Laid-Open 59-82534, Japanese Utility Model Applications Laid-Open 63-196448, 63-198473, and 63-202771 discloses systems for idling speed control systems. The systems detect instantaneous engine speeds before and after combustion in each cylinder, and the speed difference .DELTA.Ni between both engine speeds is obtained from every cylinder. The systems control engine parameters so that the speed difference .DELTA. Ni becomes zero, thereby causing the engine speed to be constant.
The systems calculate a deviation of the speed difference .DELTA. Ni from a standard value which is an average value of speed differences at all cylinders. However, the average value is subject to fluctuate in accordance with engine combustion conditions. As a result, combustion condition factors other than the detecting cylinder are included in the combustion conditions for the cylinder, so that the combustion conditions can not be accurately detected. Therefore, the idling speed is not controlled to a predetermined value, because ignition timing and the quantity of injected fuel for each cylinder are determined on the basis of the estimated combustion conditions.